1. Technical Field of the Invention
The present invention relates generally to pressure sensors. More particularly, the invention relates to a differential pressure sensor where sensor chips and pressure introduction passages are symmetrically provided to secure high accuracy of the differential pressure sensor regardless of environmental temperatures.
2. Description of the Related Art
Conventionally, there is known a type of differential pressure sensor which includes a pair of sensor chips and a circuit chip.
Each of the sensor chips works to output an electrical signal as a function of a pressure applied thereto. The circuit chip works to determine a differential pressure, which is the difference between the pressures applied to the sensor chips, based on the electrical signals output from the sensor chips. The circuit chip also works to output an electrical signal representative of the determined differential pressure. (For example, Japanese Patent First Publication No. H5-248979 discloses a pressure sensor of such type.)
Further, there is also known a structure of such type of differential pressure sensor, as shown in FIGS. 11-13.
More specifically, referring to FIG. 11, a differential pressure sensor S8 includes a case 81, a pair of sensor chips 82 and 83, an IC chip 84, capacitors 85, and a ceramic substrate 86. All the sensor chips 82 and 83, the IC chip 84, and the capacitors 85 are mounted on the substrate 86 and received along with the substrate 86 in the case 81. The case 81 has formed therein a pair of pressure introduction passages 87 and 88, through which the pressures are respectively introduced to the sensor chips 82 and 83 from external environments.
However, in the differential pressure sensor S8, referring to FIG. 12, the pressure introduction passages 87 and 88 are asymmetrically formed in the case 81. Further, referring to FIG. 13, the sensor chips 82 and 83, the IC chip 84, and the capacitors 85 are also asymmetrically arranged on the substrate 86.
Consequently, in practical use, it is difficult to secure high accuracy of the differential pressure sensor S8.
More specifically, in the differential pressure sensor S8, components made of different materials, such as the case 81 and the substrate 86, have different coefficients of linear expansion. Therefore, depending on the environmental temperatures, heat stresses may be induced in the differential pressure sensor S8. Further, due to the above-mentioned asymmetries, the heat stresses transmitted to the sensor chips 82 and 83 via the pressure introduction passages 87 and 88 are accordingly different from each other. Thus, in determination of the differential pressure by the IC chip 84, the heat stresses will not be completely cancelled by each other, resulting in a sensing error. Furthermore, due to the sensing error, a linear characteristic of the differential pressure sensor S8 can not be ensured.